Trochu nechapu logiku pouzivani Essentials. Podle mne nizsi cena nevyvazi problemy (viz napr. tento), ale budiz.
Jestli spravne chapu, staci skryt systemove mailboxy v dashboardu Essentials serveru. Viz
https://social.technet.microsoft.com/Forums/ie/en-US/b5a9a2d0-356a-4571-9854-5c0728dbb89d/essentials-2016-domain-user-accounts-exceeded-error?forum=ws16essentials